leaks many file descriptors

Functions like test_which, has_program and gather_isrcs never close devnull. Thus the leak file descriptors. This shouldn't be a problem since isrcsubmit does not open that many file descriptors. However, it would be cleaner to just surround the appropriate places with something like ```with open(os.devnull, "w") as devnull:``` to close the file once it's not needed anymore. (There's no need to add ```from __future__ import with_statement``` since Python 2.5 isn't supported anyway.)